1. Field of the Invention
The present invention relates generally to amplifiers; and more particularly, to a power sharing amplifier system for land based mobile radio systems.
2. Description of the Prior Art
T. Ha, Solid State Microwave Amplifier Design, John Wiley and Sons, 1981, pages 268-269, shows and describes a 1 by 2 balanced amplifier in Figure 6.33 thereof for amplifying a single signal. Moreover, in the prior art, 1 by N balanced amplifiers are known for amplifying a single signal, where N is greater than 2. For example, known 1 by N balanced amplifiers have a transform matrix, N shared amplifiers, and an inverse transform matrix. The transform matrix has N input ports, wherein one input port receives the signal or signals to be amplified, and all remaining input ports are connected to ground through terminating resistors and the inverse transform matrix combines the signal or signals to a single output port with the remaining output ports connected to ground through terminating resistors. In the prior art, if two or more signals are to be amplified, then a corresponding two or more 1 by N balanced amplifiers are needed and used. In this prior art, multiple input/output signal paths are not used. One disadvantage of this approach is that a separate 1 by N balanced amplifier, and separate N shared amplifiers, are needed and used for each input/output signal path which significantly increases the cost of amplifying multiple signals.
In addition, FIG. 1A shows a known cellular configuration for three cellular sectors in a land mobile radio system generally indicated as 20 with three separate low power transmitters 22, 24, 26; three separate amplifiers 28, 30, 32; and three separate antennas 34, 36 and 38. One major disadvantage of the standard cellular configuration 20 is that if an amplifier fails then a whole sector loses communication with any mobile subscriber in the sector until the amplifier is repaired or replaced. In view of this, the standard cellular configuration 20 will typically include a respective extra amplifier (not shown) for each of the three separate antennas 34, 36 and 38. Thus six amplifiers must be used. Since amplifiers are expensive, this approach increases the cost of the standard cellular configuration 20.
FIG. 1B shows a 4 by 4 cellular configuration generally indicated as 40 similar to that shown and described in U.S. patent application Ser. No. 08/683,735, which is hereby incorporated in its entirety by reference. The U.S. patent application Ser. No. 08/683,735 is commonly owned by the assignee of the present application. The 4 by 4 cellular configuration 40 has four separate low power transmitters 42, 44, 46, 48; a 4th order transform matrix circuit 50, four power sharing amplifiers 52, 54, 56, 58; a 4th order inverse transform matrix circuit 60; and four antennas generally indicated as 62, 64, 66, 68. The 4th order transform matrix circuit 50 has four input ports 50a, 50b, 50c, 50d that respond to four different input signals from the four separate low power transmitters 42, 44, 46, 48, and provides four transform matrix input signals. The four separate amplifiers 52, 54, 56, 58 respond to the four transform matrix input signals and provide four shared linear power amplified transform matrix signals. The 4th order inverse transform matrix circuit 60 responds to the four shared linear power amplified transformed signals and provides four inverse transform matrix amplified output signals to output ports 60a, 60b, 60c, 60d. In effect, the 4 by 4 cellular configuration 40 is characterized by having equal amplitude levels at all amplifiers from any input since a phase shift portion of each signal is presented to each amplifier 52, 54, 56, 58. In the amplification sharing technique used in the 4 by 4 cellular configurations 40, each amplifier 52, 54, 56, 58 shares in a substantially equal manner to the amplification of each input signal. Because of this, each shared amplifier 52, 54, 56, 58 may be four times less powerful than the amplifiers used in the standard cellular configuration shown in FIG. 1A.
FIG. 1C shows an 8 by 8 cellular configuration generally indicated as 70 similar to that shown and described in U.S. patent application Ser. No. 08/683,735. The 8 by 8 cellular configuration 70 includes eight separate low power transmitters generally indicated 72, an 8th order transform matrix circuit 74, eight amplifiers generally indicated 76, an 8 by 8 inverse transform matrix circuit 78, and eight antennas generally indicated as 80. FIG. 1D shows a mapping of each input port to a respective output port in the 8th order cellular configuration. FIG. 1E shows a chart of phase shifts for the 8th order transform matrix circuit 74 and the 8th order inverse transform matrix circuit 78 in the 8 by 8 cellular configuration.
The reader is generally referred to U.S. patent application Ser. No. 08/683,735 for a more detailed description of the 4 by 4 and 8 by 8 cellular configurations 40, 70.
The amplification sharing technique used in the 4 by 4 and 8 by 8 cellular configurations 40 and 70 offers some important advantages over the amplification technique of the standard cellular configuration 20. For example, in the 4 by 4 cellular configuration 40, the four separate amplifiers 52, 54, 56, 58 effectively share the amplification of all the input signals, which provide inherent redundancy in the overall amplification of the input signals. Thus, if one amplifier fails, then all sectors still operate and maintain communication between the subscriber and the land mobile radio system. In such a case, the 4 by 4 cellular configurations 40 may be operating in a less desirable operating mode, but nevertheless it still operates and allows additional time for the failed amplifier to be repaired or replaced. Moreover, the 4 by 4 cellular configuration 40 also uses fewer amplifiers than the standard cellular configuration 20 if redundant amplifiers are maintained for each sector. These advantages combine to make the improved cellular configuration 40 less expensive and more reliable than the standard cellular configuration 20.
However, one disadvantage of the 4 by 4 and the 8 by 8 cellular configuration 40, 70 shown in FIGS. 1B and 1C is that the amplification sharing technique results in intermodulation products between the multiple input signals to the various amplifiers that distort the amplified RF output signal sent to and transmitted by the antennas 62, 64, 66, 68, 80. The present invention provides a new and unique way to minimize the undesirable effects of the intermodulation products in the 4 by 4 and the 8 by 8 cellular configuration 40, 70, as well as other power sharing amplification systems.
In its broadest sense, the present invention provides a power sharing amplification system having an N by N transform matrix circuit, N power sharing amplifiers and an N by N inverse transform matrix circuit. The N by N transform matrix circuit has N input ports and responds to M input signals for providing N transform matrix input signals, where M is less than N. The N by N transform matrix circuit has at least one input port that does not receive an input signal. The N power sharing amplifiers respond to the N transform matrix input signals, for providing N amplified transformed signals. The N by N inverse transform matrix circuit responds to the N amplified transformed signals, for providing M inverse transform amplified output signals to N more output ports. The N by N inverse transform matrix circuit has at least one output port that does not provide an output signal to an antenna but instead provide an output for dissipation of intermodulation products. In the power sharing amplifier system of the present invention, M and N are integers greater than 1 and 2 respectively, M is less than N, M is not equal to 6, and N is not equal to 8. (A power sharing amplifier system having M equals 6 and N equals 8 for a cellular communications system is shown, described and claimed in a co-pending patent application filed concurrently with the present application and owned by the assignee of the present instant application.)
In one embodiment, the at least one input port of the N by N transform matrix circuit that does not receive an input signal is coupled to ground via a matching resistor; and the at least one output dissipation port of the N by N inverse transform matrix circuit is coupled to ground via a corresponding matching resistor to dissipate the intermodulation products.
Various embodiments are shown and described below, including a system composed of a 3rd order transform matrix with two signal inputs, three shared amplifiers and a 3rd order inverse transform matrix with two output signals referred to as a 2 by 3 amplifier system, and a 3 by 4 power sharing amplifier system (three signals and four amplifiers).
Embodiments are also described for other m by n amplifier systems including an m by 5 amplifier system, an m by 8 amplifier system, and a 6 by 8 power amplifier system (six signals and eight amplifiers) which is claimed in another related patent application.
In general, the power sharing amplifiers are configured with suitable networks at the input and output such that specific intermodulation products are xe2x80x9csteeredxe2x80x9d to specific output ports and much of the intermodulation power is dissipated in a resistive termination at the unused output port or ports.
One important feature of the claimed invention is that multiple signals are amplified with a single M by N power sharing amplifier system using the same N shared amplifiers. For example, if two or more signals are to be amplified, then the single M by N power sharing amplifier system provides the amplification for all two or more signals. One important advantage of this approach is that the use of the single M by N power sharing amplifier system to amplify multiple signals significantly decreases the cost of amplifying multiple signals, especially in the case of cellular phone applications.
Other objects of the invention will in part be apparent and will in part appear hereinafter.
Accordingly, the invention comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.